1. Field of the Invention
This invention is a further development with respect to gas-filled arc lamps. In particular, an improved electrode configuration is provided for an arc lamp in which the arc is initiated by the withdrawal of a stinger electrode from contact with a first stationary electrode with subsequent transfer of the arc to the gap between the first stationary electrode and a second stationary electrode.
2. Description of the Prior Art
The technique of initiating an arc in an enclosure filled with an ionizable gas by using a stinger electrode to draw the arc from one stationary electrode to another stationary electrode is well known. This technique has been used, for example, by Edward T. Chan in the invention disclosed in Patent application Ser. No. 337,234, filed on Mar. 1, 1973, now U.S. Pat. No. 3,876,908 Varian Associates, assignee of the present invention. In the Chan application, an electric circuit is provided whereby a low-voltage power supply can be used to start a gas-filled arc lamp wherein the arc is initiated by the withdrawal of a stinger from contact with a stationary cathode, with subsequent transfer of the arc to the gap between the cathode and a stationary anode.
In a plasma discharge arc between a cathode and an anode, the brightest portion of the arc occurs in a small region of the arc located immediately adjacent the tip of the cathode. Most of the light from the arc originates in this small region, which is called the "hot spot." In an arc lamp, in order to maximize the amount of light reflected out through the lamp window, it is necessary to fixedly position the tip of the cathode with respect to the reflector so that the hot spot remains constantly at the focus of the reflector during lamp operation. A technique for fixedly positioning the tip of the cathode with respect to the focus of the reflector is disclosed in U.S. Pat. No. 3,725,714, which issued on Apr. 3, 1973 to Norman C. Anderson and which is assigned to Varian Associates.
In an arc lamp of the type disclosed in the Chan application mentioned above, it is desirable to mount the cathode with respect to the reflector so that the tip of the cathode will remain fixedly positioned with respect to the focus of the reflector, not only during the manufacturing process but also throughout the expansion and contraction of the lamp components during the lamp's use cycle. The mounting technique disclosed in the Anderson patent mentioned above assures that the cathode itself will remain fixedly positioned with respect to the reflector during temperature cycling of the lamp. However, it has been found that repeated impacting of the stinger on the tip of the cathode tends to blunt the cathode tip, thereby changing the location of the hot spot of the arc relative to the focus of the reflector.